Mention that classpath.idx is typically generated by build plugins

Closes gh-37125

spring-boot-project/spring-boot-docs/src/docs/asciidoc/executable-jar/nested-jars.adoc

@@ -89,7 +89,9 @@ These files, however, are _not_ parsed internally as YAML and they must be writt
 [[appendix.executable-jar.nested-jars.classpath-index]]
 === Classpath Index
 The classpath index file can be provided in `BOOT-INF/classpath.idx`.
+Typically, it is generated automatically by Spring Boot's Maven and Gradle build plugins.
 It provides a list of jar names (including the directory) in the order that they should be added to the classpath.
+When generated by the build plugins, this classpath ordering matches that used by the build system for running and testing the application.
 Each line must start with dash space (`"-·"`) and names must be in double quotes.
 
 For example, given the following jar: